1. Field of the Invention
The present invention relates to a display device and a method of fabricating a display device, and more particularly, to a transflective liquid crystal display device having a retardation film, and a method of fabricating the same.
2. Discussion of the Related Art
In general, liquid crystal display (LCD) devices are classified into a transmissive type and a reflective type according to the light source used. Since the LCD device is a non-emissive type display device, an additional light source is required to illuminate the device. In a transmissive type LCD device, a backlight unit disposed under an LCD panel produces light and a transmittance of the LCD panel is adjusted according to an alignment state of a liquid crystal layer, thereby displaying images. While the transmissive type LCD device displays bright images in dark environments due to an artificial light source such as a backlight unit, the power consumption of the LCD device increases due to that backlight unit. In a reflective type LCD device, since the ambient artificial or natural light is used, power consumption of the reflective type LCD device is smaller than that of the transmissive type LCD device.
In the reflective LCD device, a reflective layer of a metallic material having a high reflectance is formed on a first substrate while a common electrode of a transparent conductive material is formed on a second substrate. Even though the low power consumption of the reflective type LCD device is improved, the reflective type LCD device may not be used where the ambient light is weak or absent.
In order to overcome such problems, a transflective type LCD device has been researched and developed. The transflective type LCD device can be switched from a transmissive mode using transmission of light to a reflective mode using reflection of light according to the user's selection. Accordingly, the disadvantages of the transmissive type LCD device and the reflective type LCD device such as high power consumption and low brightness under dark surroundings, respectively, are improved.
FIG. 1 is an exploded perspective view of a transflective type liquid crystal display device according to the related art. In FIG. 1, a liquid crystal panel 9 includes an upper substrate 24, a lower substrate 16 and a liquid crystal layer 30 interposed between the upper and lower substrates 24 and 16. A black matrix 17 is formed on the upper substrate 24 and a color filter layer 19 including sub-color filters is formed on the black matrix 17. A common electrode 22 is formed on the color filter layer 19. A reflective electrode 18, a transparent electrode 15 and a thin film transistor (TFT) “T” as a switching element are formed on the lower substrate 16 in a pixel region “P.” The pixel region “P” defined by a gate line 25 and a data line 27 includes a transmissive portion “C” and a reflective portion “D,” and the reflective electrode 18 corresponds to the reflective portion “D.” The upper substrate 24 and the lower substrate 16 may be referred to as a color filter substrate and an array substrate, respectively. The TFT “T” disposed in matrix is connected to the gate line 25 and the data line 27. In addition, the reflective electrode 18 includes one of aluminum (Al) and aluminum (Al) alloy, and the transparent electrode 15 includes one of indium-tin-oxide (ITO) and indium-zinc-oxide (IZO).
FIG. 2 is a schematic cross-sectional view of a transflective type liquid crystal display device according to the related art. For illustration of the light path, a color filter layer is omitted. In FIG. 2, a transparent electrode 15 and a reflective electrode 18 are sequentially formed on an inner surface of a lower substrate 16. A first retardation film 14 such as a quarter wave plate (QWP) and a lower polarizing plate 12 are sequentially formed on an outer surface of the lower substrate 16. In addition, a backlight unit 11 is disposed outside the lower polarizing plate 12.
A common electrode 22 is formed on an inner surface of an upper substrate 24. A second retardation film 26 and an upper polarization plate 28 are sequentially formed on an outer surface of the upper substrate 24. Further, a liquid crystal layer 30 is formed between the reflective electrode 18 and the common electrode 22 and between the transparent electrode 15 and the common electrode 22.
The first and second retardation films 14 and 26 change a polarization state of light. For example, when light passes through one of the first and second retardation films 14 and 26, a linear polarization state is transformed into a circular polarization state and a circular polarization state is transformed into a linear polarization state. The second retardation film 26 is formed to improve a polarization property in a reflective portion and the first retardation film 14 is formed to compensate for the inducement of gray due to the second retardation film 26. Polarization states of light passing through each layer of a transflective type LCD device are illustrated hereinafter.
FIGS. 3A and 3B are schematic views illustrating polarization states of light passing through a reflective portion of a transflective type liquid crystal display device according to the related art. FIG. 3A illustrates polarization states of an OFF state where an electric field is not applied to a liquid crystal layer and FIG. 3B illustrates polarization states of an ON state where an electric field is applied to a liquid crystal layer. In FIGS. 3A and 3B, the polarization states are represented by arrows when the light is observed behind.
In FIG. 3A, unpolarized light enters an upper polarizing plate 28 and only linearly polarized light having a polarization direction parallel to a polarization axis of the upper polarizing plate 28 passes through the upper polarizing plate 28. For example, the upper polarization plate 28 transmits a linearly polarized light having a polarization direction of 45° to a second retardation film 26. The linearly polarized light having a polarization direction of 45° is transformed into a left-handed circularly polarized light by the second retardation film 26 and the left-handed circularly polarized light enters a liquid crystal layer 30. Since the liquid crystal layer 30 in an OFF state has a polarization transform property (phase modulation property) of a quarter wavelength (λ/4) retardation value, the left-handed circularly polarized light is transformed into a linearly polarized light having a polarization direction of 45°. The linearly polarized light having a polarization direction of 45° is transformed into a linearly polarized light having a polarization direction of 135° while reflecting at a reflective electrode 18. The linearly polarized light having a polarization direction of 135° is transformed into a left-handed circularly polarized light while passing through the liquid crystal layer 30 and the left-handed circularly polarized light is transformed into a linearly polarized light having a polarization direction of 135° by the second retardation film 26. Since the linearly polarized light having a polarization direction of 135° is observed behind, the linearly polarized light having a polarization direction of 135° has the same polarization direction as the polarization axis of the upper polarizing plate 28. Accordingly, the linearly polarized light having a polarization direction of 135° passes through the upper polarization plate 28 and a white image is displayed.
In FIG. 3B, non-polarized light enters an upper polarizing plate 28 and only linearly polarized light having a polarization direction parallel to a polarization axis of the upper polarizing plate 28 passes through the upper polarizing plate 28. For example, the upper polarization plate 28 transmits a linearly polarized light having a polarization direction of 45° to a second retardation film 26. The linearly polarized light having a polarization direction of 45° is transformed into a left-handed circularly polarized light by the second retardation film 26 and the left-handed circularly polarized light enters a liquid crystal layer 30. Since the liquid crystal layer 30 in an ON state does not have a polarization transform property (phase modulation property), the left-handed circularly polarized light is not transformed even after passing through the liquid crystal layer 30. The left-handed circularly polarized light is transformed into a right-handed circularly polarized light while reflecting at a reflective electrode 18. The right-handed circularly polarized light is not transformed while passing through the liquid crystal layer 30. The right-handed circularly polarized light is transformed into a linearly polarized light having a polarization direction of 45° by the second retardation film 26. Since the linearly polarized light having a polarization direction of 45° is observed behind, a polarization direction of 45° for the linearly polarized light is perpendicular to the polarization axis of the upper polarizing plate 28. Accordingly, the linearly polarized light having a polarization direction of 45° does not pass through the upper polarization plate 28 and a black image is displayed.
As a result, a white image and a black image are clearly displayed in a reflective portion of the transflective type LCD device using the second retardation film 26. However, the second retardation film 26 diminishes the optical properties in a transmissive portion of the transflective type LCD device. Accordingly, an additional retardation film is required to compensate for the diminished optical properties in the transmissive portion of the transflective type LCD device.
FIG. 4A is a schematic view illustrating polarization states of light passing through a transmissive portion of a transflective type liquid crystal display device having one retardation film according to the related art and FIG. 4B is a schematic view illustrating polarization states of light passing through a transmissive portion of a transflective type liquid crystal display device having two retardation films according to the related art. FIGS. 4A and 4B show polarization states of an ON state where an electric field is applied to a liquid crystal layer in a transflective type liquid crystal display device. In FIGS. 4A and 4B, the polarization states are represented by arrows when the light is observed behind.
In FIG. 4A, non-polarized light enters a lower polarizing plate 12 and only linearly polarized light having a polarization direction parallel to a polarization axis of the lower polarizing plate 12 passes through the lower polarizing plate 12. For example, the lower polarization plate 12 transmits a linearly polarized light having a polarization direction of 45° to a liquid crystal layer 30. Because the liquid crystal layer 30 in an ON state does not have a polarization transform property (phase modulation property), the linearly polarized light having a polarization direction of 45° is not transformed even after passing through the liquid crystal layer 30. The linearly polarized light having a polarization direction of 45° is transformed into a left-handed circularly polarized light while passing through a second retardation film 26. Because the left-handed circularly polarized light has a component having a polarization direction parallel to a polarization axis of an upper polarizing plate 28, the component of the left-handed circularly polarized light passes through the upper polarizing plate 28 and a gray image is displayed. Accordingly, a white image is not clearly displayed in a transmissive portion of the transflective type LCD device having one retardation film.
In FIG. 4B, non-polarized light enters a lower polarizing plate 12 and only linearly polarized light having a polarization direction parallel to a polarization axis of the lower polarizing plate 12 passes through the lower polarizing plate 12. For example, the lower polarization plate 12 transmits a linearly polarized light having a polarization direction of 45° to a first retardation film 14. The linearly polarized light having a polarization direction of 45° is transformed into a left-handed circularly polarized light by the first retardation film 14 and the left-handed circularly polarized light enters a liquid crystal layer 30. Because the liquid crystal layer 30 in an ON state does not have a polarization transform property (phase modulation property), the left-handed circularly polarized light is not transformed even after passing through the liquid crystal layer 30. The left-handed circularly polarized light is transformed into a linearly polarized light having a polarization direction of 45° while passing through a second retardation film 26. Because the linearly polarized light having a polarization direction of 45° is observed behind, a polarization direction of 45° for the linearly polarized light is perpendicular to a polarization axis of an upper polarizing plate 28. Accordingly, the linearly polarized light having a polarization direction of 45° does not pass through the upper polarization plate 28 and a black image is displayed.
As a result, the first and second retardation films 14 and 26 are required to display a white image and a black image clearly in a transmissive portion of the transflective type LCD device. This additional retardation film complicates the fabrication process and fabrication time, thereby increasing production cost.
FIG. 5 is a schematic cross-sectional view of a transflective type liquid crystal display device according to another related art. In FIG. 5, first and second substrates “G1” and “G2” face and are spaced apart from each other. A transparent electrode 54 and a reflective electrode 52 are sequentially formed on an inner surface of the first substrate “G1” having a transmissive portion “C” and a reflective portion “D.” The reflective electrode 52 has an open portion corresponding to the transmissive portion “C” and a first retardation film 56 is formed on the transparent electrode 54 in the opening of the reflective electrode 52. A lower polarizing plate 80 is formed on an outer surface of the first substrate “G1” and a backlight unit 70 is disposed outside the lower polarizing plate 80. In addition, a common electrode 58 is formed on an inner surface of the second substrate “G2.” A second retardation film 60 and an upper polarizing plate 62 are sequentially formed on an outer surface of the second substrate “G2.” Further, a liquid crystal layer 90 is formed between the reflective electrode 52 and the common electrode 58 and between the first retardation film 56 and the common electrode 58.
A white image and a black image are clearly displayed in a transmissive portion due to the first and second retardation films 56 and 60. However, because the first retardation film 56 is formed on the transparent electrode 54, an additional photolithographic process is required. Moreover, the first retardation film may deteriorate during this additional photolithographic process. Therefore, production yield is reduced.